Detection of movement in confined spaces

ABSTRACT

A radar unit comprises a single microwave launching and receiving antenna, and a microwave oscillator feeding microwave energy to the antenna. A second oscillator is arranged to oscillate at a frequency much lower than that of the microwave energy, and includes an element common to the microwave oscillator. This common element is adapted to respond to load changes on the microwave oscillator, caused by changes in the standing wave pattern of the antenna, by varying at least one oscillation characteristic of the signal provided by the second oscillator. These changes are monitored to provide an indication of the presence and movement of objects in front of the antenna. The microwave oscillator includes a Gunn diode mounted within a resonant cavity and tuned to oscillate at the microwave frequency. An inductance connected in series with the cavity provides a series-tuned circuit of the second oscillator, the cavity providing the common element. The lower frequency oscillations are limited to lie within a predetermined range to improve the sensitivity of the arrangement. This may be done with a feedback circuit. Three such units may be mounted on a common support so as to lie in respective vertical planes at 120* to one another, their combined responses overlapping to provide unbroken coverage in horizontal plane surrounding the common support. The aerials may be shaped to have lobes such that the unbroken coverage extends to a range of some 30 feet from the support.

Waited States Wtent l loice [54] DETECTION OF MOVEMENT IN [21] Appl. No.: 107,808

[30] Foreign Application Priority Data May 3, 1970 Great Britain.....'.....26,858/70 July 28, 1970 Great Britain ..36,52l/70 [52] U.S. Cl. ..343/5 PD, 340/258 A, 340/258 B, 343/7.7 [51] Int. Cl ..G01s 9/42, G08b 13/24 [58] Field of Search ..343/5 PD, 7.7; 340/258 A, 258 B [56] References Cited UNITED STATES PATENTS 3,041,592 6/1962 Schmidt ..343/5 PD X 2,859,435 11/1958 Auer, Jr. et al ..343/s 3,126,539 3/1964 Mulvey ..343/s 3,317,912 5/1967 Kelleher ..343/840 x FOREIGN PATENTS OR APPLICATIONS 656,399 8/1951 Great Britain ..340/258 A Primary Eirar n iner- -lvialcolm F. I-lubler Attorney-Mellin, Moore & Weissenberger [57] ABSTRACT A radar unit comprises a single microwave launching and receiving antenna, and a microwave oscillator feeding microwave energy to the antenna. A second oscillator is arranged to oscillate at a frequency much lower than that of the microwave energy, and includes an element common to the microwave oscillator. This common element is adapted to respond to load changes on the microwave oscillator, caused by changes in the standing wave pattern of the antenna, by varying at least one oscillation characteristic of the signal provided by the second oscillator. These changes are monitored to provide an indication of the presence and movement of objects in front of the antenna. The microwave oscillator includes a Gunn diode mounted within a resonant cavity and tuned to oscillate at the microwave frequency. An inductance connected in series with the cavity provides a seriestuned circuit of the second oscillator, the cavity providing the common element. The lower frequency oscillations are limited to lie within a predetermined range to improve the sensitivity of the arrangement. This may be done with a feedback circuit. Three such units may be mounted on a common support so as to lie in respective vertical planes at 120 to one another, their combined responses overlapping to provide unbroken coverage in horizontal plane surround- .ing the common support. The aerials may be shaped to have lobes such that the unbroken coverage extends to a range of some 30 feet from the support.

6 Claims, 9 Drawing Figures PATENTEDsP12|s12 3 691 556 INVENTOR JOHN ANTHONY BLO W P ATTORNEYS PATENTEDSEP 12 I972 SHEET 2 BF 6 F/G.3. Y

INVENTOR JOHN ANTHONY BLOICE BY W, W Y WW ATTORNEY PATENTED E 1 2 I972 SHEET 3 0F 6 HIM INVENTOR JOHN ANTHONY BLOICE BY maww w' ATTORNEYS PATENTEDsEP 12 I972 SHEET t 8F 6 INVENTOR JOHN ANTHONY BLOICE BY M M WW ATTORNEYS PATENTED SEP 12 I912 SHEEI 5 [IF 6 NmI w F a 5 NB a M 2a E RNN WQNN R aw i 3 v 5 5 WW 5 5 xx INVENTOR JOHN ANTHONY BLOICE ATTORNEY PATENTEDsEP 12 m2 3 69 1. 556

sum 6 0F 6 IN V E NTOR JOHN ANTHON Y BLOICE BY W M W ATTORNEYS DETECTION OF MOVEMENT IN CONFINED SPACES BACKGROUND OF THE INVENTION The present invention relates to improvements in or relating to the detection of movements in confined spaces, and is concerned with apparatus for this purpose. The invention provides radar apparatus using microwave oscillator circuits.

It has already been proposed to use a Gunn diode in the dual role of transmitter and receiver by operating it in the so-called oscillator mixer mode. The Doppler principal is employed, with the result that a stationary object cannot be detected.

A Gunn diode employed as an oscillator mixer has poor sensitivity, since while a satisfactory source of microwave energy it is not satisfactory as a mixer device. It is generally necessary to pass up to 200 milliamps through the device to maintain a reasonable output power level, and a detected signal will be a fraction of milliamp. For this signal to appear as a voltage, the voltage must be developed across a resistor. The operating current of the Gunn diode must also pass through this resistor, whose value cannot exceed a few hundred ohms if the voltage drop developed by the operating current is not to be excessive and power wastage considerable.

As the Gunn diode is a negative resistance device, a high value series resistor will cause unstable operation. A resistance value of the order of 50 to 100 ohms is thus generally chosen. Even so, this will result in appreciable power loss and require a higher operating voltage than may otherwise be desirable.

The signal voltage is developed across this resistor, and is unlikely to exceed a few millivolts. As there will be several volts dropped across the resistor from the load current, and only a few millivolts of signal, the smoothness and stability of the power supply would need to be very high indeed. Even a power supply fluctuation of one-tenth of 1 percent would provide a signal" of the same order of magnitude as the genuine signal.

Also, the Gunn diode is substantially a constant current device when working at its operating point, and a large variation in supply voltage causes little variation in current flow. This effect will tend to reduce the efficiency of the device as a detector since the voltage drop across the load resistor will tend to stay at a constant level despite additional signal voltages developed in the Gunn diode.

SUMMARY OF THE INVENTION In one aspect the invention provides an electronic oscillator circuit comprising first oscillator means providing an oscillatory fist output signal, first load means fed with the oscillatory first output signal from said first oscillator, a second oscillator means providing an oscillating second output signal at a frequency different to said first output signal, second load means fed with said oscillatory second output signal, an interoscillator coupling element common to both oscillators so that an impedance change in said first load means of said first oscillator means causes said element to respond by varying a characteristic of said oscillatory second output signal provided by second oscillator means.

The first oscillator is preferably a resonant cavity oscillator, and suitably comprises a Gunn diode mounted in a cavity tuned to resonate at a microwave first frequency.

The exact mechanism of the interaction between the first and second oscillators is not exactly understood, but it is believed that they interact in the following way, with particular reference to an arrangement in which the first oscillator comprises a Gunn diode mounted in a microwave resonant cavity.

The Gunn diode is a substantially constant current device having a negative resistance characteristic. When mounted in a microwave resonant circuit such as a tuned cavity, the microwave oscillator circuit as a whole will present a negative resistance characteristic. The circuit can thus support further oscillations externally of the microwave circuit, and these have generally been suppressed by a shunt capacitance. By using this characteristic in a second oscillator operating at a frequency different from the microwave oscillator, it has been found that virtually undetectable changesin the microwave oscillation conditions as the load impedance on the microwave oscillator changes produce substantial changes in the oscillation conditions in the second oscillator.

It appears that the changes in the microwave power due to the microwave load impedance variations, while being virtually negligible as compared with the total power fed to the microwave oscillator, because of the distribution of oscillator power between the microwave oscillator and second oscillator, result in substantial changes in the power fed to the second oscillator.

The resulting changes in oscillation level in the second oscillator are readily monitored to provide an indication of the change in load conditions of the microwave oscillator. The sensitivity may be improved by carefully controlling the level of the oscillations in the second oscillator.

The second oscillator suitably comprises a close coupled series-tuned circuit with the microwave oscillator forming one of the series elements of the circuit.

The microwave oscillator may be used to provide microwave power for radar apparatus. By using the second oscillator to monitor load conditions on the microwave oscillator, a simple radar can be produced using a single aerial and without complex microwave plumbing.

Using the invention, it is possible to provide a radar apparatus using one aerial eliminating the often complicated circulator or directional coupler by eliminating the conventional receiving element from the radar. This receiving element usually comprises a diode on which reflected radiation is directed.

In another aspect, the invention provides a radar unit comprising: a microwave launching and receiving antenna; a microwave first oscillator tuned to oscillate at a microwave first frequency and arranged to feed microwave energy to said antenna; a second oscillator arranged to oscillate at a second frequency different to said first frequency; an inter-oscillator coupling element common to said first and second oscillators and adapted to respond to impedance load changes on said first oscillator by varying at least one oscillation characteristic of the oscillatory signal of said second oscillator; and circuitry for monitoring the oscillatory signal of said second oscillator, whereby changes in the standing wave pattern of said antenna caused by objects in front of said antenna are detected by monitoring the variations in the oscillatory signal of said second oscillator.

Unlike the Doppler system, a stationary object introduced into the field of view of the radar apparatus causes a permanent change in the loading of the oscillator and can thus be detected. A change in position of the target will again change the loading on the oscillator, whose output level will change and hold a new value until the target moves again.

By using variations in the impedance loading on the Gunn diode, stationary as well as moving targets can be detected by the radar apparatus, as opposed to Doppler radar apparatus where only moving targets are detected. Nevertheless, the new oscillator arrangement may be used with a Doppler system in which a Gunn diode is used as an oscillator mixer detector to increase the sensitivity of the Gunn diode, the minute current changes being monitored in the second oscillator.

The single aerial radar apparatus produces a radiation patter in the form of a lobe which is excellent for screening areas through which an intruder must necessarily come.

In another aspect, the present invention provides radar apparatus comprising three similar outwardly directed single aerials such as just described, mounted on a common support and lying respectively in vertical planes at substantially 120 to one another, each aerial having a radiation pattern which is of flattened lens shape in the vertical plane and of substantially circular shape in the horizontal plane, the three horizontal circles overlapping one another to provide unbroken horizontal coverage over a radial distance extending to a range approaching 30 feet from the support.

The invention will now be described in more detail, by way of examples only, with reference to the accompanying diagrammatic drawings.

IN THE DRAWINGS FIG. 1 is a partly cut-away plan view of a radar scanner unit incorporating a cheese aerial;

FIG. 2 is a front elevation of the scanner unit partly cut away;

FIG. 3 is a partly cut-away side elevation of the scanner unit; unit;

FIG. 4 is a first form of circuitry for the scanner unit;

FIG. 5 is a second form of circuitry for the scanner unit;

FIG. 6 shows a third form of circuitry for the scanner unit;

FIG. 7 shows a fourth form of circuitry for the scanner unit;

FIG. 8 shows the radiation pattern of the cheese aerial of the scanner unit when arranged with its longer side vertical and when arranged with its longer side horizontal; and

FIG. 9 shows apparatus utilizing three scanning units providing complete all-round cover in the horizontal plane from a distance of approximately 30 feet from the apparatus to a point close to the apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 to 3, a radar apparatus comprises a Gunn diode microwave oscillator 1 from which microwave energy is coupled into a parabolic cheese aerial 2 through a quarterwave stub 3. The oscillator 1 is mounted directly on the aerial 2, which consists of an aluminum casting. Also mounted on the aerial is a circuit board 4 carrying power supply components of the microwave oscillator l and the components of a second oscillator. The electrical circuitry of the radar will be described in more detail below.

The oscillator 1 and circuit board 4 are enclosed by a housing 5 which fits onto the aerial 2 by means of a tongue and groove arrangement seen most clearly in FIGS. 2 and 3. The housing 5 is secured to the aerial by two screws 6 and 7 passing through a fascia 8 of the housing 5 into respective lugs 9 and 10 on the horn 2. A readily detachable panel 1 1 conceals the screws once the housing 5 is in position.

The power supply to the radar and leads to external monitor circuitry are provided by a cable 12 passing through the housing 5 and secured by a clamp 13. The radar is provided with an indicator lamp 14, mounted on the fascia 8.

The top of the housing 5 carries a hinged mounting plate 15 with which the radar can be mounted in a suitable position on a wall or like support. The plate 15 is secured to the housing by a nut and bolt 16, 17 respectively.

The opening of the aerial 2 is closed by a plate 18 of expanded polystyrene which is transparent to microwave radiation, giving negligible reflections. The shape of the aerial 2 is such that a flat radiation curtain extends away from the aerial in the plane of FIG. 1. The axis of the hinged plate 15 is parallel to the plane of this radiation curtain and to the mouth of the aerial.

The Gunn diode 40 is mounted in a resonant cavity 41 formed within an aluminum housing 42. A front plate 28 is bolted to the housing 42 and insulated from it by a sheet of electrically insulative PTEE material 32. A positive terminal 33 for the Gunn diode is mounted in an aperture in the front plate 28. The diode 40 is sandwiched between the positive terminal 33 and a copper bar 44 providing the negative terminal. The end of the bar 44 remote from the Gunn diode 40 is attached to the center of a flexible diaphragm 45 made of beryllium copper and clamped between an end plate 46 and the housing 42 to which the end plate is bolted at 47. The diaphragm allows a thermal expansion and contraction of parts associated with the Gunn diode 40 so that it is not unduly stressed as a result of thermal expansions and contractions of the parts associated with it.

The oscillatory cavity 41 of the Gunn diode is capacitance coupled to a pin 50 which passes through a hollow screw 51 to provide with its other end the quarter-wave stub 3 transmitting radiation into the aerial 2. The pin 50 is suitably insulated so that the coupling assembly forms a short length of coaxial line. The screw 51 also serves to attach the cavity assembly to the aerial.

FIG. 4 shows a simple form of the electrical circuitry of the radar apparatus. The microwave oscillator and aerial are shown at 30, the positive and negative terminals at 33 and 34 respectively. The positive terminal 33 is connected to the positive terminal '63 of a unidirectional voltage source providing an output voltage of seven volts, through an inductance 64. The negative terminal 34 is connected to a point at earth potential. A semi-conductor diode 65 has is anode connected to the terminal 34. A capacitance 66 is connected between the terminal 33 and the cathode of the diode 65. The anode and cathode of the diode 65 are also connected to the emitter and base respectively of an NPN transistor 67. The collector of the transistor 67 is connected to the terminal 63 through a milliammeter 68.

The insulative material 32 between the plate 28 and the oscillator assembly provides a direct current break in the resonant cavity. The inductance 64 is designed to have a resistance of less than 1 ohm and has a value which, in combination with the capacitance value of the direct current break provides a second oscillator giving an oscillation external to the cavity at a frequency of, for example, 1 MHz. The cavity acts as an interoscillator coupling element common to this second oscillator and the microwave oscillator.

Two separate frequencies of oscillation are maintained by the Gunn diode. One is within the microwave cavity and is in the microwave band of frequencies. The other is external to the cavity and has the value of 1 MHz.

The higher frequency provides the energy coupled to the aerial 8 which provides a radiation curtain whose configuration depends on the shape or form of the aerial. This radiation is reflected from objects in front of the aerial and some radiation is returned to the aerial, setting up standing wave patterns within the aerial. These standing wave patterns change when the objects reflecting energy to the horn move in relation to the horn.

The changing wave patterns within the aerial give rise to changes in the oscillation characteristics, that is amplitude and/or frequency, of the second oscillator output, which are monitored by the milliammeter 68. Sensitivity is increased by carefully controlling the level of this external oscillation, by means of the diode 65 and transistor 67 which, for silicon devices, limit the peak to peak voltage at the base of the transistor to 1.5 volts. Positive going pulses exceeding 0.75 volts are conducted to earth by the base emitter junction of the transistor, and negative going pulses exceeding 0.75 volts are conducted to earth by the diode 65.

As the amplitude of the external oscillation changes, so does the collector current of the transistor 67, and by monitoring this change in current with the milliammeter 68 the changing oscillation conditions externally of the microwave cavity may be observed. The meter indicates a rise and fall in current as an object approaches the aerial, with peaks at half wave-length spacings. The presence of a stationary object is indicated by a permanent change in the level of current.

In the modified circuit shown in FIG. 5, the inductance 64 is replaced by an auto-transformer providing the inductance value for tuning the external oscillations and also stepping up or down the voltage to be applied to the monitoring circuitry.

Referring to FIG. 5, the cavity and aerial are indicated at 70. A 12 volt direct current supply is connected to positive and negative terminals 71 and 72 respectively. A zener diode 73 has its anode connected to terminal 72 and its cathode connected to terminal 71 through a resistance 74. The zener diode is shunted by a variable resistance 75 and resistance 76 connected in series. The moving contact of resistance 75 is connected to the base of an NPN transistor 77 and is connected to earth through a capacitance 78. The collector of transistor 77 is connected to terminal 71 and the emitter through a capacitance 79 to terminal 72.

The positive terminal of the cavity 70 is connected to one end of a tapped inductance 80 forming an autotransformer and whose other end is connected via a capacitance 81 to the cathode of a diode 82. The tap of the inductance 80 is connected to the emitter of transistor 77.

The cathode of diode 82 is connected to the base of a second NPN transistor 83 whose emitter is connected to the terminal 72. The collector is connected through a resistance 84 shunted by a variable resistance 85 to the emitter of transistor 77.

The collector of transistor 83 is also connected to the terminal 72 through a capacitance 86.

The moving contact of variable resistance 85 is connected through a capacitance 87 to the base of a third NPN transistor 88 whose emitter is connected to the terminal 72. The collector is connected through a resistance 89 to the emitter of transistor 77, and to the base through a series combination of resistances 90 and 91 shunted by a capacitance 92. The junction of resistances 90 and 91 is connected to earth through an electrolytic capacitance 93.

The collector of transistor 88 is also connected through an electrolytic capacitance 94 to the base of a fourth NPN transistor 95 and the cathode of a diode 96 whose anode is connected to the terminal 72.

The emitter and collector of transistor 95 are connected to the terminal 72 directly and through an electrolytic capacitance 97 respectively. The collector is connected through a resistance 98 to the terminal 71, and through a resistance 99 to the base of a fifth NPN transistor 100.

The emitter of transistor 100 is connected to the terminal 72 and the collector through a series combination of an electrolytic capacitance 301 and a resistance 302 to the base. The junction of capacitance 301 and resistance 302 is connected to one terminal of lamp 303 whose other terminal is connected to the terminal 71. The lamp 303 is arranged to illuminate a photo-sensitive resistance or cadmium sulphide cell 304.

The transistor 77 and the circuitry connected to its base act as a voltage regulator, providing a steady voltage of about 7.5 volts at the emitter of the transistor from the 12 volt supply. The cavity is connected to the emitter via part of the inductance 80, and the voltage on the cavity is set to an optimum value by the variable resistor 75.

The inductance 80 tunes the external oscillations of the cavity and provides a step up of the voltage applied to the diode 82 and transistor 83 which limit the oscillation level as do the diode 65 and transistor 67 of the circuit shown in FIG. 5.

The resistance 84 is chosen on test to give the required amount of voltage swing on the collector of transistor 83.

The voltage from the movable contact of variable resistance is coupled through capacitance 87 to a conventional amplifier circuit including transistor 88. The capacitance 92 provides a selective degenerative feedback from the collector to the base, reducing the gain at high frequencies. The frequencies applied to the base of transistor 88 due to movements in front of the aerial are of the order of l to 50 cycles per second.

The output of this amplifier circuit is rectified and amplified by the diode 96 and transistor 95, whose output is connected to transistor 100 which acts as a lamp driver.

In the quiescent condition, transistor 100 is conducting and the lamp 303 is illuminated. The collector voltage of transistor 9 is near the supply voltage, since this transistor is not conducting. When a movement signal appears at the base of the transistor 95, the latter begins to conduct.

The collector voltage falls, cutting off transistor 100 and extinguishing the lamp 303. The capacitance 301 introduces a delay of the order of 250 milliseconds, so that the lamp is not extinguished by transients whether of electrical origin or caused, for example, by an insect flying across the aerial aperture.

The use of a lamp and photosensitive element provides an electrical output with no direct electrical connection. In addition, the lamp can provide a visual indication of movement in front of the aerial, providing the indicator lamp 14 of FIGS. 1 to 3.

The circuit just described is sensitive to fluctuations in temperature and supply voltage. The circuit of FIG. 6 uses feedback to eliminate this disadvantage, and also to regulate the external oscillation level.

Referring to FIG. 6, the microwave cavity and aerial are shown at 101. The negative terminal of the cavity is connected to the earth line and the positive terminal to one end of an inductance 102 whose other end is connected to the emitter of an NPN transistor 103 whose collector is connected to the positive terminal 104 of a unidirectional voltage source providing 12 volts.

A resistance 105 has one end connected to the terminal 104 and its other end to the cathode of a zener diode 106 whose anode is connected in the earth line. The cathode of the zener diode 106 is connected to the positive terminal of the cavity through a resistance 107 and capacitance 108, and also to the collector of a second NPN transistor 109. The junction of the resistance 107 and capacitance 108 is connected to the base of the transistor 109 and the cathode of a diode 111 whose anode is connected to the emitter of the transistor 109 and to the earth line through a capacitance 112. The emitter of the transistor 109 is connected to the earth line through a variable resistance 113 and a capacitance 114, the junction of this resistance and capacitance being connected to the base of the transistor 103.

A third NPN transistor 115 has its base connected to the sliding contact of the variable resistance 113 through a capacitance 1 16. Its collector is connected to that of transistor 100 through a resistance 117 and to its own base through a resistance 118. The emitter is connected to the earth line.

The collector of the transistor 115 is also connected to the earth line through a capacitance 121 and through a capacitance 122 in series with a diode 123 whose anode is connected to the earth line. The cathode of this diode 123 is connected to the base of a fourth NPN transistor 124.

The emitter of transistor 124 is connected to the earth line; the collector is connected to the terminal 104 through a resistance 125, to the earth line through a capacitance 126, and to the base of a fifth NPN transistor 127. The emitter of transistor 127 is connected to the earth line and the collector to terminal 104 through a lamp 128. The lamp is arranged to illuminate a photo-sensitive resistor or cadmium sulphide cell 129.

OPERATION This circuit operates as follows The microwave output of a Gunn diode oscillator optimizes at a particular voltage, typically 7 volts. Generally the output drops off more sharply when the supply is reduced below the optimum value than when the supply is raised above it.

The maximum sensitivity of the arrangement occurs when the low frequency oscillation is maintained at a low level. The maximum level of low frequency oscillation may not necessarily occur at the same voltage as that required to maintain the maximum level of microwave oscillation.

It would be possible to either raise or lower the voltage about that required for maximum level of low frequency oscillation to reduce the level by the required amount.

However, it will be seen that it is preferable to increase the voltage on the cavity to reduce the level of low frequency oscillation as this will result in a smaller drop in microwave power than would occur if the voltage were to be decreased.

The circuit of FIG. 6 prevents this by means of a feedback system. A portion of the low frequency oscillation is rectified, amplified and employed to further increase the supply voltage supplied to the cavity. This process continues until the increasing voltage on the cavity starts to reduce the level of low frequency oscillation as the voltage rises above the optimum level for that oscillation. A state of equilibrium will be reached where a further increase in low frequency level will increase the supply voltage and therefore reduce it and vice versa. The circuit therefore achieves two purposes. It maintains the low frequency oscillation at a low level for maximum sensitivity and compensates for changes in supply voltage, temperature, and so on.

On connecting the 12 volt supply, resistance and zener diode 106 will provide a stabilized supply of 9 volts to the collector of transistor 109. Resistance 107 will pass a small current into the base of transistor 109 causing it to conduct. The emitter current passes through variable resistance 113 into capacitor 1 14. The values of resistance 113 and capacitance 114 provide a time constant of a second or so. Therefore the base of transistor 103 will rise from zero volts and the emitter will also rise slowly on connecting the supply. An increasing voltage will be developed across the cavity and at a certain point external low frequency oscillations will commerce to develop across the cavity and inductance 102. This signal is fed via coupling capacitor 108 to the rectifying circuit formed by diode 111 and transistor 109 causing the latter to conduct harder and further increase the drive to transistor 103 and hence the supply of power to the cavity. This process will continue until at a certain point the low frequency oscillations will begin to diminish and a state of equilibrium will be reached with the low frequency oscillations maintained at a low but stable level.

A movement in front of the aerial will cause corresponding changed in the level of low frequency oscillations, as fairly rapid fluctuations of the order of l to 50 per second caused by such movements are prevented from being fed back to transistor 103 by capacitance 114. Long term changes due to supply fluctuations or temperature changes, however are fed back and employed to maintain a stable state of low frequency oscillation Fluctuations in the low frequency oscillation caused by a moving object are rectified by diode 111 and transistor 109 and the signal developed across variable resistance 113. Capacitance 112 shunts low frequency oscillations to earth.

Transistor 115 amplifies the voltage on the moving contact of variable resistance 113. Any high frequency noise originating in the amplifier stage is shunted by capacitance 121. The amplified signal is rectified by the circuit consisting of diode 123 and transistor 124.

Transistor 127 drives the lamp 120. In the quiescent condition the lamp supply is ensured by the resistance 125. When the signal supplied to transistor 124 reaches a sufficiently high value, the transistor conducts, reducing the base current of transistor 127 and extinguishing the lamp 128. Capacitance 126 prevents transients causing spurious operation of the lamp driver.

The lamp provides a visual indication of the presence of an object in front of the radar aerial. The lamp, or another in parallel with it can form the indicator lamp 14 of FIGS. 1 to 3. In addition, the photo-resistor 129 gives an electrical indication as its resistance increases rapidly when its illumination is cut off. This arrangement of lamp 120 and cell 129 provides an electrical output without a direct electrical connection between the output and the rest of the circuitry.

Referring to FIG. 7, the microwave cavity 201 has its negative terminal connected in the earth line and its positive terminal connected through an inductance 202 to the emitter of an NPN transistor 203 whose collector is connected to the positive terminal 204 of a 12 volt direct current supply. A series combination of a resistance 205 and a zener diode 206 is connected between terminal 204 and the earth line, the. anode of the diode 206 being connected to the earth line. Its cathode is connected through a series combination of a resistance 207 and capacitance 208 to the positive terminal of the microwave cavity, and to the collector of another NPN transistor 209.

The junction of resistance 207 and capacitance 208 is connected to the base of the transistor 209 and to the cathode of a diode 211 whose anode is connected to the emitter of the transistor 209 and to the earth line through a variable resistance 213 shunted by a capacitance 212. The emitter of the transistor 209 is connected to the base of the transistor 203. A third NPN transistor 215 has its base connected to the slider of the variable resistance 213 through a capacitance 216 and its collector connected to that of transistor 209 through a resistance 217. The collector and base are connected together by a resistance 218. The emitter is connected directly to the earth line and the collector is connected to the earth line through a capacitance 221. The collector is also connected to the earth line through a series combination of a capacitance 222 and diode 223 whose anode is connected to the earth line. The cathode is connected to the base of a fourth NPN transistor 224 whose emitter is connected to the earth line and whose collector is connected to terminal 204 through a resistance 225, to the earth line through a capacitance 226 and to the base of a fifth NPN transistor 227. The emitter of transistor 227 is connected to the earth line and the collector to terminal 204 through a lamp 228 arranged to illuminate a photosensitive resistor or cadmium sulphide cell 229.

The circuit operation is very similar to that of the circuit of FIG. 6, but the feedback is applied instantaneously to the cavity from transistor 203. Movements in front of the aerial do not cause fluctuations in the low frequency oscillations whose amplitude is held constant. Instead, the variations in the level of feedback are monitored by the variable resistance 213 and amplified by the rest of the circuit which is identical to the corresponding part of the circuit of FIG. 6. The capacitance 212 shunts low frequency oscillations to the earth line.

The overall dimensions of the scanner unit shown in FIGS. 1 and 2 is as follows:

Rectangular front (FIG.2) =4W'xx 2.5%" Front to Back (FIG. 1) 3%" The cheese aerial has an aperture width of one-half inch and a length of 4% inches.

As shown in FIG. 8 the radiation pattern with the longer dimensions of the cheese aerial vertical is effectively a circle of about 30 feet diameter. If the cheese aerial is held horizontal the radiation pattern has the flattened lens shape shown to the right of FIG. 8.

These polar diagrams shown in FIG. 8 enable the apparatus shown in FIG. 9 to be used to provide all around coverage from a distance of a few inches in front of the aerials to a range of 30 feet or thereabouts.

In FIG. 9 is shown a ceiling beam 200 to which is attached a support post 201 carrying at its lower end a bracket 202 having three flanges 203 on each of which is supported a scanner unit 204 of the form previously described and as shown in FIGS. 1 and 2. The three cheese aerials thus lie respectively in three vertical planes immediately displaced by to one another. The three circular polar diagrams in the horizontal plane produced by the three aerials intersect one another to provide the above-mentioned horizontal coverage.

The apparatus shown in FIG. 9 is well suited to provide continuous surveillance over large enclosed areas such as drawing offices.

I claim: and receiving 1. A radar unit comprising: a microwave launching and receiving antenna; a microwave first oscillator tuned to oscillate at a microwave first frequency and arranged to feed microwave energy to said antenna, said first oscillator including means defining a resonant cavity and a Gunn diode disposed within said cavity; a

second oscillator arranged to oscillate at a second frequency different to said first frequency, said second oscillator including a series-tuned circuit comprising an inductance connected in series with said resonant cavity, said means constituting an inter-oscillator coupling element common to said first and second oscillators and adapted to respond to impedance load changes on said first oscillator by varying at least one oscillation characteristic of the oscillatory signal of said second oscillator; and circuitry of monitoring the oscillatory signal of said second oscillator, whereby changes in the standing wave pattern of said antenna caused by objects in front of said antenna are detected by monitorin g the variations in the oscillatory signal of said second oscillator.

2. A radar unit as set forth in claim 1, wherein said inductance comprises an auto-transformer connected to change the voltage of the oscillatory signal of said second oscillator and to supply the changed voltage signal to said circuitry for monitoring the oscillatory signal of said second oscillator.

3. A radar unit as set forth in claim 1, including circuitry for limiting the amplitude of the oscillatory signal of said second oscillator.

4. A radar unit as set forth in claim 3, wherein said circuitry comprises a feedback circuit including a rectifier circuit connected to receive a portion of the oscillatory signal of said second oscillator, an amplifier connected to receive the rectifier circuit output, and current control means connected between said first oscillator and its power supply to receive the amplifier output and to control the microwave oscillation in accordance therewith.

5. Radar apparatus comprising three radar units each including: a microwave launching and receiving antenna; a microwave first oscillator tuned to oscillate at a microwave first frequency and arranged to feed microwave energy to said antenna, said first oscillator including means defining a resonant cavity and a Gunn diode disposed within said cavity; a second oscillator arranged to oscillate at a second frequency different to said first frequency, said second oscillator including a series-tuned circuit comprising an inductance connected in series with said resonant cavity, said means constituting an inter-oscillator coupling element common to said first and second oscillators and adapted to respond to impedance load changes on said first oscillator by varying at least one oscillation characteristic of the oscillatory signal of said second oscillator; and circuitry for monitoring the oscillatory signal of said second oscillator; said radar units being outwardly directed and mounted on a common support, being so arranged in use as to lie in respective vertical planes at substantially to one another, each unit having a radiation pattern which is of flattened lens shaped in the vertical plane and of substantially circular shape in the horizontal plane, so that the three horizontal circles overlap one another to provide unbroken coverage in a horizontal plane surrounding the position of mounting of the apparatus.

6. Radar apparatus as set forth in claim 5, wherein the antennae of said radar units are shaped to define radiation patterns intersecting to provide substantially unbroken coverage extending to a radius of some 30 feet from said support. 

1. A radar unit comprising: a microwave launching and receiving antenna; a microwave first oscillator tuned to oscillate at a microwave first frequency and arranged to feed microwave energy to said antenna, said first oscillator including means defining a resonant cavity and a Gunn diode disposed within said cavity; a second oscillator arranged to oscillate at a second frequency different to said first frequency, said second oscillator including a series-tuned circuit comprising an inductance connected in series with said resonant cavity, said means constituting an inter-oscillator coupling element common to said first and second oscillators and adapted to respond to impedance load changes on said first oscillator by varying at least one oscillation characteristic of the oscillatory signal of said second oscillator; and circuitry of monitoring the oscillatory signal of said second oscillator, whereby changes in the standing wave pattern of said antenna caused by objects in front of said antenna are detected by monitoring the variations in the oscillatory signal of said second oscillator.
 2. A radar unit as set forth in claim 1, wherein said inductance comprises an auto-transformer connected to change the voltage of the oscillatory signal of said second oscillator and to supply the changed voltage signal to said circuitry for monitoring the oscillatory signal of said second oscillator.
 3. A radar unit as set forth in claim 1, including circuitry for liMiting the amplitude of the oscillatory signal of said second oscillator.
 4. A radar unit as set forth in claim 3, wherein said circuitry comprises a feedback circuit including a rectifier circuit connected to receive a portion of the oscillatory signal of said second oscillator, an amplifier connected to receive the rectifier circuit output, and current control means connected between said first oscillator and its power supply to receive the amplifier output and to control the microwave oscillation in accordance therewith.
 5. Radar apparatus comprising three radar units each including: a microwave launching and receiving antenna; a microwave first oscillator tuned to oscillate at a microwave first frequency and arranged to feed microwave energy to said antenna, said first oscillator including means defining a resonant cavity and a Gunn diode disposed within said cavity; a second oscillator arranged to oscillate at a second frequency different to said first frequency, said second oscillator including a series-tuned circuit comprising an inductance connected in series with said resonant cavity, said means constituting an inter-oscillator coupling element common to said first and second oscillators and adapted to respond to impedance load changes on said first oscillator by varying at least one oscillation characteristic of the oscillatory signal of said second oscillator; and circuitry for monitoring the oscillatory signal of said second oscillator; said radar units being outwardly directed and mounted on a common support, being so arranged in use as to lie in respective vertical planes at substantially 120* to one another, each unit having a radiation pattern which is of flattened lens shaped in the vertical plane and of substantially circular shape in the horizontal plane, so that the three horizontal circles overlap one another to provide unbroken coverage in a horizontal plane surrounding the position of mounting of the apparatus.
 6. Radar apparatus as set forth in claim 5, wherein the antennae of said radar units are shaped to define radiation patterns intersecting to provide substantially unbroken coverage extending to a radius of some 30 feet from said support. 